1. Field of the Invention
The present invention relates to a rotor for an automotive alternator, which has a Lundell-type field core, for mounting on an automobile engine, and in particular, relates to a winding configuration for winding onto the Lundell-type field core.
2. Description of the Related Art
FIG. 6 is a cross-section of a conventional rotor for an automotive alternator and FIG. 7 is a cross-section of part of the rotor shown in FIG. 6.
In FIGS. 6 and 7, a rotor 1 comprises a rotating shaft 11 rotatably supported by a pair of brackets (not shown), a pair of Lundell-type field cores 12a, 12b secured to the rotating shaft 11, a pair of fans 13a, 13b secured to both axial ends of the field cores 12a, 12b, slip rings 14 secured to one end of the rotating shaft 11, and a field winding 15 wound onto the field cores 12a, 12b. 
The field cores 12a, 12b are made of iron, comprise cylindrical base portions 121a, 121b fitted over and secured to the rotating shaft 11 and claw-shaped magnetic poles 122a, 122b plurally projecting from the outer circumferential edges of the base portions 121a, 121b, and are secured to the rotating shaft 11 facing each other such that the end surfaces of the base portions 121a, 121b are in close contact with each other and the claw-shaped magnetic poles 122a, 122b intermesh alternately. The field winding 15 is a copper wire with a circular cross-section and is wound a predetermined number of times onto a bobbin 16 fitted over the outer circumferences of the base portions 121a, 121b. A magnetic flux is generated when an electric current is supplied to the field winding 15 by means of the slip rings 14 and magnetic poles are formed in the field cores 12a, 12b by the magnetic flux.
Inner circumferential tape 17a for protecting the winding is wound onto the cylindrical portion 16a of the bobbin 16. Outer circumferential tape 17c for protecting the winding is also wound onto the outer circumference of the field winding 15 wound onto the bobbin 16. In addition, side tape 17b is disposed between the lead portion of the field winding 15 and the multi-layered portion of the field winding 15.
The construction of the field winding 15 will now be explained with reference to FIGS. 8 to 10.
The bobbin 16 is made of resin, and as shown in FIG. 8, comprises a cylindrical portion 16a and a pair of first and second annular flange portions 16b projecting perpendicularly from both ends of the cylindrical portion 16a. A recessed groove 161 for housing a lead wire 15a at the start of the winding is disposed radially in the inner surface of the first flange portion 16b so as to extend from the outer circumferential side thereof to the cylindrical portion 16a. An anchor portion 16c is disposed on an outer circumferential portion of the first flange portion 16b in close proximity to the upper end of the recessed groove 161.
First, the inner circumferential tape 17a is wound onto the cylindrical portion 16a of the bobbin 16. Then, the starting portion of the field winding 15 is wound around the anchor portion 16c, inserted into the recessed groove 161, and drawn from the lower end (inner circumferential end) of the recessed groove 161 onto the cylindrical portion 16a. At this point, the side tape 17b is pasted onto the inner surface of the first flange portion 16b so as to cover the lead wire 15a at the start of the field winding 15 which is housed in the recessed groove 161. Then, as shown in FIG. 9, the field winding 15 drawn out onto the cylindrical portion 16a is lined up in rows at an angle xe2x80x9caxe2x80x9d relative to a plane that perpendicularly intersects the axial center of the cylindrical portion 16a as it is wound onto the cylindrical portion 16a. Then, as shown in FIG. 10, when the first layer of the winding is finished, a second layer is lined up in rows at an angle xe2x80x9cbxe2x80x9d relative to the plane that perpendicularly intersects the axial center as it is wound onto the cylindrical portion 16a. In this way, the field winding 15 is wound up layer by layer in order from the bottom of the cylindrical portion 16a, and when a predetermined number of layers have been wound, the outer circumferential tape 17c is wound onto the outermost circumferential portion. In addition, the multi-layered portion of the field winding 15 is saturated with varnish. For example, when a field winding 15 is wound onto the cylindrical portion 16a of a bobbin 16 with an outer diameter of 40 to 60 mm, the outermost diametric dimension of the multi-layered portion on which the outer circumferential tape 17c is wound is approximately 70 to 90 mm.
In the rotor 1 constructed in this manner, centrifugal force acts constantly on the field winding 15 during power generation, and even slight gaps and looseness are gradually enlarged, leading to disarray in the winding. Thus, in order to achieve winding without gaps or looseness, it is usual to apply tension to the wire as it is wound onto the bobbin 16, and with respect to the configuration of the start of the winding, to line up the winding in rows at an angle relative to the plane which perpendicularly intersects the axial center as shown in FIG. 9.
In the field winding 15 wound in this manner, the winding in the second layer, for example, is wound on top of the winding in the first layer with the angle reversed. Thus, the winding configuration in the field winding 15 assumes a first condition in which the portions of wire in the second layer are positioned in the exact center between the adjacent portions of wire in the first layer (condition in FIG. 11 which is a cross-section taken along line Qxe2x80x94Q in FIG. 10), a second condition in which the largest diameter portions of the wire in the first layer and the wire in the second layer are stacked radially (condition in FIG. 12 which is a cross-section taken along line Pxe2x80x94P in FIG. 10), and intermediate conditions which gradually shift from the first condition to the second condition or from the second condition to the first condition. At that time, the height t1 of the two layers in the second condition is greater than the height t of the two layers in the first condition.
In a conventional rotor for an automotive alternator constructed in this manner, the field winding 15, which has a circular cross-section, is wound onto the bobbin 16 at an angle to a plane which perpendicularly intersects the axial center, and therefore a first condition in which the portions of wire in the nth+1 layer are positioned in the exact center between the adjacent portions of wire in the nth layer, a second condition in which the largest diameter portions of the wire in the nth layer and the wire in the nth+1 layer are stacked radially, and intermediate conditions which gradually shift between those conditions.
Thus, one problem is that radial irregularities invariably arise within each lap of the field winding 15 and the configuration of the multi-layered portion thereof consequently has eccentricities, which increases vibrations during high-speed rotation, leading to bending of the rotating shaft 11 or disconnection of the winding connections.
Another problem is that the space factor in the multi-layered portion reaches a peak, precluding increases in output.
An additional problem is that portions of wire in the second condition are in contact with other portions at points, making resistance to vibrations poor and giving rise to disarray in the winding. By touching the root portions of the claw-shaped magnetic poles 122a, 122b, the outside of the multi-layered portion of the field winding 15 serves the role of damping axial vibrations in the claw-shaped magnetic poles 122a, 122b, and therefore disarray in the winding leads to increased electromagnetic noise.
The present invention aims to solve the above problems and an object of the present invention is to provide a rotor for an automotive alternator enabling elimination of eccentricities in the multi-layered portion of the field winding and reduction of vibrations during high-speed rotation, and enabling increases in space factor, increases in output, increases in rigidity, and reductions in electromagnetic noise, by winding a flattened field winding onto the bobbin with the flat surfaces as the inner circumferential surface and the outer circumferential surface relative to the radial direction.
In order to achieve the above object, according to one aspect of the present invention, there is provided a rotor for an automotive alternator comprising a pair of field cores each having a cylindrical base portion and a plurality of claw-shaped magnetic poles projecting from the outer circumferential edges of the base portions, the field cores being secured to a rotating shaft facing each other such that the end surfaces of the base portions are in close contact with each other and the claw-shaped magnetic poles intermesh with each other; a cylindrical bobbin having a cylindrical portion and a pair of first and second annular flange portions projecting perpendicularly from both ends of the cylindrical portion, the bobbin being fitted over the base portions of the pair of field cores; and a field winding wound a predetermined number of turns into multiple layers on the cylindrical portion of the bobbin, wherein the field winding has a flat shape in which a pair of opposite flat surfaces are parallel, the field winding being wound onto the cylindrical portion of the bobbin such that the pair of opposite flat surfaces face the inner circumferential side and the outer circumferential side, respectively, relative to the radial direction.